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| Research on Thermal-Oxidative Stability of Polypropylene Cable for High Operating Temperature |
| Zhang Lizhi1, Tao Xiantao2, Xing Yunqi1, Zhou Linan1, Gao Sixuan1 |
1. School of Electrical Engineering Hebei University of Technology Tianjin 300131 China;
2. China Electric Power Research Institute Wuhan 430074 China |
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Abstract As a thermoplastic material, polypropylene (PP) can be recycled and has excellent electrical and thermal properties. It has gradually replaced cross-linked polyethylene as a power cable insulation material to start engineering applications. However, PP has the disadvantages of poor low temperature impact resistance, high hardness, high crystallinity and low toughness. Therefore, when it is used for cable insulation, some grades of PP materials have been modified, such as blending modification, copolymerization modification and grafting modification. Thermal oxygen aging occurs during cable operation, which reduces insulation performance and shortens cable service life. Currently, the aging law of the PP cable needs to be further studied. Moreover, the dielectric constant and dielectric loss of PP cable are higher than those of XLPE cable because of the different molecular structures. That causes the loss heating of PP cable to be more serious than that of XLPE cable. Additionally, the long-term working temperature of PP materials can reach 100℃~120℃, and increasing the operating temperature of PP cable can improve its current carrying ampacity (IA). But the specific increase range and the thermal oxygen stability of PP cable at high operating temperature need to be further studied.
Firstly, the IA of XLPE and PP cables was calculated according to IEC 60287. The results show that when at 90℃, the IA of the 110 kV and 35 kV XLPE cables is 1 871.46 A and 888.96 A, and that of PP cables is 1 811.488 A and 858.49 A, respectively. Compared with the IA of the XLPE cables, the value of PP cables decreases by 3.40% and 3.46%. However, when at 100℃,the IA of the 110 kV and 35 kV PP cables rises to 1 901.08 A and 913.13 A, which is 2.35% and 1.75% higher than that of the XLPE cables at 90℃, respectively. Hence, the current carrying ampacity of PP cables can be increased by raising the operating temperature to 100℃.
To further verify the feasibility of long-term operation of PP cables at 100℃, six commercial XLPE and PP cables were selected as experimental samples. The samples were placed in ovens with 100℃, and the aging time were set to 250 h, 500 h and 750 h. Differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were practiced testing and analyze the crystallinity, grain size, thermal decomposition characteristics and life expectancy of PP and XLPE cable samples in each aging stage. The results show that the crystallinity (Xc) and the grain size (L) of XLPE and PP increase firstly and then decrease during the process of thermal-oxygen aging, suggesting samples have experienced two stages of recrystallization and thermal oxidative degradation. Specifically, during the recrystallization process, the PP cables display significantly smaller increases on the Xc (by 42.58%) and the L (by 76.26%) compared to those of the XLPE cables, respectively. Similarly, during the thermal-oxidative degradation process, the reduction of the PP cables in Xc and L is relatively reduced by 60.19% and 45.22% than those of the XLPE cables. Moreover, in the two stages, the change of Xc of PP-grafted cables was 17.68% and 16.04% smaller than that of PP-pure cables, while the change of L was also reduced by 40.17% and 60.34%, respectively. The change of Xc of PP-blened cables was 26.03% and 13.76% bigger than that of PP-pure cables, while the change of L was also increased by 141.88% and 84.48%, respectively. In addition, the variation of thermal decomposition temperature of PP cable is also smaller, which is 81.33% lower than that of XLPE cable. Hence, it can be inferred that the thermo-oxidative aging resistance of PP cable is better than that of XLPE cable in 100℃, and the performance of PP-grafted cable is the best.
Finally, the results of thermal aging life expectancy show that the predicted life of the PP cable at 100℃ is still more than 80 years, which is basically the same as that of the XLPE cable at 90℃, suggesting that PP cable can stably operate at 100℃. Therefore, the maximum operating temperature of PP cable can be set to 100℃ to obtain higher current carrying ampacity.
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Received: 08 August 2024
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2025, Vol. 40 
